Zeolite-like gallosilicates ("ZAGs") crystallographically are tectosilicates and have a structure which is built up from TO.sub.4 tetrahedra that are connected via the oxygens. In the case of the ZAGs, the T atoms represent either quadrivalent silicon or trivalent gallium. These TO.sub.4 tetrahedra form chains and layers and these, in turn, build up defined cavity systems including ducts and pores with opening widths of molecular dimensions. The opening widths of the ducts and pores determine the accessibility to the internal cavity structure or materials in accordance with their shape and form. As a result, the porous structures have molecular sieve properties. Due to the incorporation of trivalent gallium, the crystalline lattice of the zeolite-like gallosilicates (ZAGs) has an excess negative charge, which is compensated for during synthesis by the presence of cations (usually alkali or alkaline earth ions).
If the alkali or alkaline earth ions are exchanged after the synthesis for protons, effective acidic catalysts which are useful for heterogeneous catalysis are obtained. Because of their molecular sieve properties, the gallosilicate catalysts of this invention have shape-selective properties.
However, the selectivity and, particularly, the activity of the ZAGs are determined not only by their crystalline features or their pore structure, but also by the size of the crystals and the accessibility of the active centers. "Active centers" are strong Broenstedt-sites (bridged hydroxyl groups) which are connected to gallium tetrahedral atoms inside the lattice or weak Lewis-sites connected either to "extra-framework" gallium species or to silanol groups. To achieve the best possible action of the active centers that are present in the crystal, these centers should be distributed as uniformly as possible throughout the crystal.
The presently known ZAGs are synthesized by a hydrothermal process using organic compounds, generally ammonium compounds, which have structure-directing and structure stabilizing functions (as discussed, for example, in European patent application EP 0 327 189 A2). These compounds are often referred to as "templates."
The synthesis methods heretofore used for synthesizing ZAGs have a number of serious disadvantages which preclude their operation on a large industrial scale without contaminating the environment. The templates used (generally tetraalkylammonium compounds) are toxic, easily inflammable and highly corrosive. Since the synthesis is a hydrothermal reaction that is carried out under high pressure, the escape of these template materials cannot be prevented completely. There is therefore a high potential for endangering the employees and the environment both near and far from the production site. The effluent resulting from the synthesis also contains these template materials and must therefore be treated and disposed of at high cost to prevent environmental contamination. A further disadvantage is the need to burn out (i.e., calcine) the organic components in the lattice at high temperatures. As a result, the templates or their decomposition or breakdown products reach the waste air and must be removed by expensive filtering methods. Calcining can also damage the lattice structure of the ZAGs, adversely affecting their catalytic and adsorptive properties. Moreover, calcining can lead to mechanical damage to the ZAGs.
In addition to the cost and environmental danger resulting from the use of templates, the broad distribution of particle sizes resulting from known synthesis methods is also disadvantageous. This broad distribution of particle sizes decreases the stability and useful life of the ZAGs, as measured by their catalytic properties. Moreover, known ZAGs have an unsatisfactorily inhomogeneous distribution of active centers, which unfavorably affects the selectivity and conversion rate for catalytic reactions.